Development and spatial distribution of soils on an active volcano:
Mt Etna, Sicily
Peter James
a,
, David K. Chester
a,b
, Angus M. Duncan
a
a
Department of Geography and Planning, School of Environmental Sciences, University of Liverpool, Liverpool, L69 3BX, UK
b
Department of Geography and Environmental Studies, Liverpool Hope University, Hope Park, Liverpool L16 9JD, UK
abstractarticle info
Article history:
Received 11 June 2015
Received in revised form 21 September 2015
Accepted 30 September 2015
Available online 22 October 2015
Keywords:
Mt Etna
Soils
Pedogenesis
Chronosequence
Lava
Tephra
As a large, active volcano, Mt Etna presents a complex soil-forming environment, with a spatial diversity of lava-
ow age, tephra deposition, local climate and agriculture. Following a review of previous published research on
Etna's soils, this paper analyses physical and chemical data for 23 proles between 120 m and 1030 m elevation,
on all anks of the volcano, with the aim of characterising the soils and determining the extent to which the chief
controls on their nature and geography may be identied. Soils are sampled across a chronosequence of lava
ows, dating from 72 yr to between 28 ka and 42 ka BP, which occur in conditions of diverse land use and
local climate. Ages of ows N 3000 yr are only broadly constrained. The majority of soils comprise mostly tephra.
Chief soil processes inferred have been identied in previous published research on Mt Etna: accumulation of soil
organic matter, rapid loss of Na and production of ne-grained materials, particularly those inferred to be of
short-range order. Other developments are soil structural formation and evidence of increasing soil mesofaunal
activity. Despite the complexity of environmental diversity and spatial variation in tephra deposition, a number
of soil properties (including: material b.020 mm; organic C; organic N; C/N; pH; total Na; extractable phases of Fe
and Al) are statistically related to age of lava ow, most of these also related to annual rainfall. That a number of
statistically signicant relationships with age of ow also hold for near ground-surface samples is unexpected
and unexplained. Two areas which require further research are the precise impacts of tephra deposition on de-
velopment of Etnean soils, and processes of weathering and related pedogenesis at the soillava bedrock
interface.
© 2015 Elsevier B.V. All rights reserved.
1. Introduction and aims
This paper reviews previous research into the soils of the largest ter-
restrial volcano on the European mainland, the active Mt Etna (Fig. 1),
and analyses new data. Because of their global distribution, the human
settlements and economies they support, their distinctive properties
and signicance as an element of the volcanic environment, volcanic
soils have attracted much attention from specialists in pedology, ecolo-
gy, hydrology, environmental chemistry, horticulture and archaeology.
Soils may also contribute to the impact of volcanoes on the atmosphere:
rapid weathering of volcanic materials, a part of pedogenesis, may con-
stitute an important sink in the global CO
2
cycle (Vitousek et al., 1997).
Distinguishing between release of soil-derived CO
2
and the higher
degassing rates of magma-derived CO
2
is an important aid in predicting
volcanic activity (Liuzzo et al., 2013). Soil reactive processes moderate
storage and release to groundwater of volcanigenic elements potentially
harmful to humans (Bellomo et al., 2007; Floor et al., 2011).
The genesis of volcanic soils is reasonably well understood (for a
summary of chief processes, see Supplementary Appendix 1). Specic
process pathways, development rates and geography of soils on a par-
ticular volca no, however, are not readily predicted. Large, active
volca noes, especially those of predominantly Hawaiian/Strombolian
eruptive style such as Mt Etna, present some of the most complex soil-
forming environments on Earth. Historic activity on Mt Etna has been
predominantly Strombolian in style, but periodic phases of la va
fountaining of Hawaiian style generate considerable lapilli fall on the
anks. In the historic period there have been major explosive eruptions
including the plinian event of 122 BC which deposited up to 15 cm of
tephra on Catania some 3 0 km from the vent (Coltelli et al., 1998).
Several subplinian eruptions have also occurred (Branca and Del Carlo,
2005). Diversity in age and character of volcanic materials, land surface
morphology, local climate, vegetation and land use history translates
into complex soil spatial patterns determined by controls operating at
scales of differing spatial order, e.g. local climate, age and morphology
of lava ow, and even individual plants (Certini et al., 2001). In the ver-
tical dimension (i.e. prole), complex soils result fr om intermittent
tephra deposition, anthropogenic disturbance, erosion and subsequent
deposition. Soil proles may reect the amount and frequency of tephra
deposition as much as normal prole-forming soil processes operating
Catena 137 (2016) 277297
Corresponding author.
E-mail addresses: jg04@liverpool.ac.uk (P. James), jg54@liverpool.ac.uk (D.K. Chester),
aduncan@liverpool.ac.uk (A.M. Duncan).
http://dx.doi.org/10.1016/j.catena.2015.09.023
0341-8162 2015 Elsevier B.V. All rights reserved.
Contents lists available at ScienceDirect
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on stable surfaces. The building-up of soil by the addition of tephra may
have either a retardant or developmental effect on pedogenesis
(Johnson, 1985). In the former case existing soil is effectively sealed
by burial, and pedogenesis begins anew in fresh material; in the latter,
pedogenesis keeps pace with a lesser amount of added tephra, absorb-
ing the new material, the soil nevertheles s being altered and made
more difcult to interpret. A problem in dealing with the impact of
tephra deposition is its considerable spatial va riation, because the
Fig. 1. Mt Etna. a. Relief, geology and location of sampling sites. Other units include basal tholeiites and pyroclastic deposits. b. Mean annual rainfall and wind direction (after Durbin and
Henderson-Sellers, 1981); chief settlements and place-names referred to in text. c. Map of Fierotti et al.'s (1988) soil associations. d. Location of Mt Etna and study area.
278 P. James et al. / Catena 137 (2016) 277297
history of deposition at a particular location is unlikely to be known.
Furthermore, units of tephra in soil are not everywhere easily differen-
tiated in the eld.
Many analyses of soil spatial variation on volcanoes relate soil prop-
erties to environmental gradients which represent important state fac-
tors in pedogenesis. The gradients are sequences of age, climate (as it
varies with elevation and slope orientation), vegetation or topography.
Many volcanic environments are well suited to chronosequence studies
because of the age-range of lava surfaces. Examples abound. Vitousek
et al. (1997) applied the chronosequence approach to the study of soil
and ecosystem development across a well-dened 1.4 m yr sequence
in Hawaii. Shorter age-ranges have been examined in Italy: e.g. on Mt
Amiata by Lulli (2007), and in three studies comprising the most com-
prehen sive published analyses of soil proles on Mt Etna to date, by
Egli et al. (2007, 2008, 2012). Drawbacks of soil chronosequence model-
ling include the role of factors other than time and the problem of cli-
mate change over longer timescales (Huggett, 1998). There is also the
problem of determining age of landforms and/or sediments. Age esti-
mates for older (N 3000 ka) lava ows on Mt Etna are only broadly
constrained (De Beni et al., 2011). Where appropriate and with careful
interpretation, however, chronosequence studies provide at least a
good basis to begin analysis of processes and patterns of soil develop-
ment across an area.
Ped ological research on Mt Etna has tended to co ncen trate either
on selected soil properties or processes, or on relationships between
soils and certain vegetation types or plant spec ies, and deals in most
cases w ith comp rehensive analyses of a small numb er of soil proles.
This approach is essential to understanding the soils and the envi-
ronmental controls upon them. Of equal importance, however, are
the soils' spatial relationships with environmental controls as they
vary across the volcano. There are two small-scale soil maps of Mt
Etna (Comitato per la Carta dei Suoli, 1966; Fierotti et al., 1988),
but without accompanying published details on the soils. Egli et al.
(2007, 20 08, 2012) nd that a number of soil properties are related
to age of the substrate lava and to elevation (and therefore precipita-
tion, temperature and vegetation). In the present study we cast the
spatial net more widely, examini ng a sample of soils on all anks of
the volcano, and across a range of environments, inc luding those dis-
turbed by agriculture, and across a range of local climates. The soil at-
tributes selected for analysis are fundamental to soils generally and
to volcanic soils particularly. Our aim is to determine whether, and
to what exten t, environmental controls on pedogenesis and soil spa-
tial distrib ution are identiable from the complex of inuences af-
fecting the soils sampled. This is attempted using statistical tests
and by considering spati al variation in soil characteristics. Soil vari-
ables are rst plotted against age of lava ow. The ows comprise a
chronosequence. The soils in a strict sense do not: they have devel-
oped thr ough a range of past environmental conditions. Age of ow
is, nevertheless, a sound basis for soil sampling, and the question of
the extent to which soil properties reect ow-age remains one to
be addressed.
Soils are described using the terminol ogy of Schoeneberger et al.
(2012). They are classied only tentatively, and as far as the derived
data permit, according to the Soil Taxonomy system (Soil Survey Staff,
1999).
Most Soil Taxonomy terms used in the text are dened in the
glossary provided in the Supplementary Appendix 2. For further details
and for terms not included in the glossary, the reader is referred to the
Soil Survey Staff (1999, 2014).
2. Mt Etna
Volcanism in the Etnean region started during the middle Pleisto-
cene, at about 600 ka (Branc a et al., 2004). The vo lcano now spans
3350 m in elevation and 40 km in basal diameter; tephra fall has been
frequent. Below 1100 m above sea level (a.s.l.), the sampling zone (i.e.
1201030 m), lava varies in age from the 2014/15 ow to the ca
500,000 yr-old tholeitic basalts of a small area on the lower part of the
southern ank (Gillot et al., 1994; De Beni et al ., 2011). Flows of the
last millennium (F ig. 1a), referred to below as historical, are dated
using written records; older (prehistoric) ows by archaeomagnetic
and radiometric, particularly U series, dating (Condomines et al., 1982;
Tanguy et al., 2007, 2012). Terrain of historical and of some prehistoric
ows is dominated by rugged aa lava, pahoehoe and toothpaste
morphology
1
(Kilburn and Guest, 1993; Polacci and Papale, 1999;
Guest et al., 2012). The mineralogy and geochemistry of historical and
young prehistoric Mt Etna lavas vary little (Romano and Guest, 1979:
see Table 1; Tanguy et al., 1997; Corsaro and Pompilio, 2004). The
hawaiite lavas are porphyritic with up to 45% phenocrysts comprising
plagioglase, clinopyroxene and olivine (typically in that order of abun-
dance) set in a matrix of microlites and glass with scattered
titanomagnetite granules (Chester et al., 1985). Explosive activity from
summit craters is frequent. Less frequent eruptions, often with higher
effusion rates, occur from ank vents, with Strombolian activity from
vents high on the volcano (Chester et al., 1985). The geochemistry of
the tephra, emitted from summit craters and lateral vents, is similar to
that of the lava (Coltelli et al., 2000). The tephra (which consists of phe-
nocrysts set in a glassy matrix) varies in deposition rate and particle-
size with distance and direction from source, and accumulates unevenly
on rugged lava surfaces. The detailed tephra chronostratigraphy at any
location is thus likely to be unique and unknown. Heiken and Wohletz
(1985) reported the media n particle-size of tephra samples from a
1971 eruption to vary between .001 and 1 mm. There are few tephra de-
posits comprising extensive stratigraphic markers (Del Carlo and
Branca, 1998). In the soils we examine, only one buried tephra layer is
ident i ed, that of the 1669 eruption of the Monti Rossi cinder cones
(Fig. 1b). Tephra at our site ES19, on the 1792 ow, is probably from a
recent eruption.
A Mediterranean climate of marked seasonality is recorded at a
number of stations on Mt Etna (Carpenter, 2004). Mean annual rainfall
(m.a.r.) increases with elevation and is signicantly greater on the E
than that on t h e W ank (Fig. 1b):
Adrano, W ank, 589 m. a.s.l.: m.a.r. 497 mm; January means: 69 mm,
9.8 °C; July m eans: 14.3 mm, 26 °C.
Zafferana, E ank, 574 m a.s.l.: m.a.r. 1353 mm; January means:
227 mm, 9.2 °C; July means: 17 mm (June: 12.7 mm), 24.8 °C.
A strong prevalence of westerly to northwesterly winds (Fig. 1b;
Durbin and Henderson-Sellers, 1981)issignicant for the distribution
of airborne volcanic emissions.
Judging from the meteorological data and high permeability of
many soils, soil moisture regimes for the study area are xeric and
ustic; and temperature regimes predominantly thermic, with mesic
at the higher elev ations (see the Supplementary Appe ndix 2 for def-
initions). Fernández-Sanjurjo et al. (2003) judge the regimes at
Monti Rossi (850 m a.s.l., Fig. 1a) to be ustic and mesic. Geobotanical
regions of Mt Etna range with elevation through thermo, meso, supra
and oromedi terranean (Poli Marchese and Patti, 2000), the sampling
area for the present study lying within the rst two.
Below 900 m elevation the more productive soils are terraced for a
wide range of tree crops, vines and to a more limited extent for arable
crops (Dazzi, 2 007). Some terraces, especially at higher elevations, have
not been cultivated for a numb er of years, t hough these are s till grazed.
1
aa is lava where the surface crust of active lava ow is disrupted into irregular frag-
ments of clinker that are rafted on the mobile interior of the ow. Surfaces comprise blocks
in an openwork structure. Pahoehoe typically has smooth, undulating surfaces, sometimes
lobate or rucked up forming a ropy texture. Toothpaste is transitional between aa and
pahoehoe. It has a viscosity higher than that of pahoehoe and rate of ow slower than aa.
As its name implies, it has a morphology that resembles toothpaste extruded from a tube.
279P. James et al. / Catena 137 (2016) 277297
Woodland is common on older owsbetween900and2000ma.s.l.,acli-
mate cont rol evident in a preference of evergreen oak (Quercus i lex)for
the W side and deciduous oak (Quercus pubescens; Quercus cerris)and
economically impor tant chestnut (Castanea sativa) woods for the E side
of the mountain (Poli Marchese and Patti, 2000; Puzzolo and Folving,
2001). The E tna broom (Genista aetnensis), a common colonizer of open
areas, is found up to 1800 m a.s.l. Major ash fall may cause signicant
damage to fruit and vegetable crops ( Dazzi, 2007). Within our sampling
zone, broad relationships between vegetation on the one hand and age,
morphology a nd slope aspec t of lava ow on the other, are v isi ble in the
landscape (as reected in the site data provided in Supplementary
Table 1). At the sites examined, the Etna broom occurs on ows older
than that of 1923; 3 m-high deciduous oak (Quercus sp.) on the 1865
ow; 3.5 m everg reen oak (Q. ilex) on the 17 92 ow; mature deciduous
oak woodland on the 1408, and mixed deci duous woodl and on prehist or-
ic ows. Agricultural terraces or walled enclosures occur at ve of the sites
(671 yr- old), and intense grazing at two (Supplementary T able 1).
Using palynological data from Lago di Pergusa in central Sicily
(667 m a.s.l., 70 km to the west of Mt Etna), Sadori and Narcisi (2001)
infer that the transition from dry Pleistocene to more moist Holocene
climate started at about 10,700 yr BP. The wettest conditions of the Post-
glacial occurred between ~9000~7200 yr BP, followed by a trend to-
wards desiccation at about 3000 yr BP. Signicant human impact upon
vegetation and soils of Mt Etna is likely to have begun early, during
the Bronze Age, if not in Neolithic times (Leighton, 1999; Chester
et al., 2010).
In a few of the many papers published on the chemistry of gas and
particle emissions, rainwater and groundwater, implications are consid-
ered for rock weathering but not for pedogenesis. Degassing from the
summit craters continuously releases volatiles into the atmosphere,
with high enrichment of at least 13 trace metals together with Al, Fe,
CO
2
and SO
2
in the volcani c plume (Buat-Ménard and Arnold, 1978;
Varekamp et al., 1986; Francis et al., 1998; Gauthier and le Cloarec,
1998; Varrica et al., 2000; Aiuppa et al., 2003a, 2003b). Mt Etna is the
world's greatest point source of F, SO
2
and HCl (Allard, 1997; Francis
et al., 1998), b 1% being deposited around the volcano during eruptions
(Aiuppa et al., 2006). Rapid downward mixing of the plume on to the
lower anks of the volcano, however, occurs under certain meteorolog-
ical conditions (Allen et al., 2006). Particulate emissions also contribute
signicantly to rainwater chemistry (Cimino and Toscano, 1998),
deposition rates of major ions being controlled mainly by rainfall
(Aiuppa et al., 2006). Na, K, Ca and Mg have varied sources, including
volcanic ash and aerosols. Volcanic emissions comprise a signicant ad-
dition to dust sources of many chemical elements in lichens on Mt Etna
(Varrica et al., 2000). Vacca et al (2003) and Egli et al. (2008) conclude,
however, that an allochthonous loessic component is of minor impor-
tance in volcanic soils of southern Italy. Passive degassing on Mt Etna's
anks results in intense leaching of basalt by phreatic groundwaters
rich in CO
2
(Giammanco et al., 1996; Aiuppa et al., 2000; 2004;
Alparone et al., 2004). The residence time of any gas-charged waters
in permeable soil is short, but the effect of magma-derived CO
2
on Mt
Etna soils has not to our knowledge been researched.
3. The soils of Mt Etna: review of research
Part of Fierotti et al.'s (1988) map is reproduced in Fig. 1c. In this,
soils are classied according to Soil Survey Staff (1975).Associations1
and 10, largely at higher elevations, were classed as Lithic, Typic and
Andic Xerorthents, and Andic Xerochrepts; Association 15 at lower ele-
vations, and extensive on E, S and SW anks, included Typic
Xerorthents, Andic Xerochrepts and Ultic Haploxeralfs. In the scheme
of Soil Survey Staff (1975) a separate Andisol Order had not been intro-
duced (they were in 1978); the soils classed by Fierotti et al. as andic
would probably include soils now classed as Andisols. The Ultic
Haploxeralfs, the most fully developed of the soils mapped, are referred
to as clayey and as havin g ABtCprole which includes well struc-
tured, organic-rich, dark brown or black A horizon and blocky, more
often prismatic, argillic B horizon. Dazzi (2007) and Lo Papa et al.
(2003) (both referred to by Lulli, 2007), encountered only young, vitric
soils on Mt Etna, Lo Papa nding Typic Vitrixerands, Typic Udivitrands
and Vitric Hapludands, all under forest (and all Andisols).
Published papers analyse pedological aspects (Bellanca et al., 1996;
Buemi et a l., 1998; Busà et al., 1998; Certini et al., 2001 and Parisi et al.,
2003); effects of plants on soils (Certini et al., 2001; Fernández-Sanjurjo
et al., 2003) and biogeochemical cycling (Leonardi and Rapp, 1981;
Leonardi et al., 1992; Leonardo et al., 1996; Santa Regina et al., 2001). Min-
eralization of carbon in soils of Mt Etna was analysed by Hopkins and
Bartoli (2004), and soil microbial activity by Hopkins et al. (2007) and
Shillam et a l. (2008). Plant colonisation and lichen-weathering of lavas
have been examined by Carpenter (2004).
In rece nt studies on or near the cinder cones on the south ank of
the volcano, soils classie d by variou s authors include: sand-skeletal,
mixed, mesic, aridic Ustipsamment (beneath broom on tephra of
Monti Rossi, 850 m a.s.l., Fig. 1c); sand-skeletal, mixed, frigid
Vitrandic Udorthent (under broom and pine Pinus nigra on
Monte Vetore, 1820 m a.s.l., Fig. 1c) and Vi tric Hapludands (elevation
not given; precipitation, 1130 mm yr
1
)(Bellanca et al., 1996; Busà
et al., 1998; Del Carlo and Branca, 1998; Palumbo et al., 2000; Certini
et al., 2001; Fernández-Sanjurjo et al., 2003). The weakly weathered
Vitric Hapludand reported by Palumbo et al. (2000) contains allo-
phane, imogolite and halloysite. Andic properties do not occur in
the Monti Rossi and Mt Vetore soils described by Fernández-
Sanjurjo et al. (2003),butwereconrmed by Bel lanca et al. (1996)
in a soil in which allophane, imogolite, halloysite and a small amount
of goethite were evident in the B horizon. The former authors de-
scribe different characteristics between soil under the canopy of
bro om and that outside the canopy on Monti Rossi. The sand fraction
of these soils is lar gely vesiculated gl ass, with plagioclase, augite,
magnetite and apatite. Organic matter and NH
4
oxalate-extractable
Al, Si and Fe increased, and Ca, Mg, K and Si de creased down-
prole, reecting greater alteration of tephra in the B than in the A
horizon formed in overlying younger tephra. Busà et al. (1998) also
describe a reduction in Ca, Mg and K, and relative enrichment in Al,
Fe and Ti concentrations at 1372 m a.s.l. near the Monte Salto del
Cane cone (Fig. 1c). Contrasts between soil proles on Mt Vetore
(1800 m a.s.l.) and Monti Rossi (850 m a.s.l.) are attributed by
Fernández-Sanjurjo et al. (2003) to differences in climate and more
intense pedogenesis associated with plant roots at the lower site.
Certini et al. (2001) conside r the greater acid weathering under
pine than under Etna broom on Mt Vetore to be a cause for concern
regarding loss of soil fertility, as t he Corsican pine (P. nigra)has
been widely planted on Mt Etna in recent deca des. Those authors at-
tribute low amounts of N in the Vitrandic Udorthent of Mt Vetore in
part to coarse mineral texture and continual soi l rejuvenation by
fresh cinder. Total C, microbial biomass and microbial respiration
were analysed by Hopkins et al. (2007) in eight soils under trees be-
tween 850 m and 1 450 m a.s.l. Carbon content and microbial biom ass
were g reater in more developed soils, but ability to analyse trends
with soil development was limited by differences in management
Table 1
Selected oxides (percent weight) for Etna lavas, dating from 1923, 1947, 1809 and 161424.
Romano and Guest (1979).
Min Max
SiO
2
46.33 48.86
Al
2
O
3
17.35 18.94
Fe
2
O
3
5.48 7.06
FeO 3.01 5.82
MgO 4.44 5.85
CaO 9.53 10.37
Na
2
O 3.32 4.40
K
2
O 1.50 1.76
280 P. James et al. / Catena 137 (2016) 277297
and tephra addi tion. Shillam et al. (2008) compared the stru ctural
and functional properties of the microbial communities in two soils
of different ages on Mt Etna. The microbial community in the older
soil was structurally more diverse than t hat of the younger soil but
proved in experiments not to be more resistant to environmental
disturbance simulated by experimental heating.
The important role of reactive surfaces of ne materials of Mt Etna
soils on the storage and release of volcanigenic pollutants to groundwa-
ter has been demonstrated. Bellomo et al. (2007) estimate that soils ab-
sorb ~60% of atmospheric F depositions, maintaining F within the safe
drinking-water limit; the ac tive soil constituents being short-range
order minerals (SROMs), particularly allophane. Both SROMs and soil
organic matter are implicated by Floor et al. (2011) for Se immobiliza-
tion, but with competition by sulphate for adsorption sites and mineral
dissolution by acid rainwater (on the E ank) accounting for release of
adsorbed Se to groundwater (with less Se mobilized in organic-rich
soils).
Egli et al. (2008) examined eight soils (6 on the N, 2 on the SE ank)
between 780 and 1090 m a.s.l., on lavas 125 to 115,000 yr old and with
estimated mean annual rainfalls ranging between 1100 and 1200 mm.
Only soils on non-agricultural land were sampled. The soils, classied
using IUSS Working Group WRB (2006), were equivalent to Entisol on
the 125-yr old lava, Inceptisols on 358 and 438 yr BP lavas and Andisols
on 115,000 yr BP lavas. The main mineral transformations were inferred
to be volcanic glass imogolite -type minerals kaolinite. Textures
were predominantly sand and loamy sand. These authors found the
chief pedogenic processes to be leaching of base cations and passive en-
richm ent of Al and Fe, and that these were pronounced in the early
stages of pedogenesis, with much reduced rate of cha nge between
15,000 and 115,000 yrs. In contrast, the rate of formation of imogolite-
type minerals and particularly of ferrihydrite was more linear through
the chronosequence. Imogolite was transformed to kaolinite, amounts
of 2:1-type clay minerals being very small even on the oldest ows,
and inferred to have been released from glass by weathering rather
than resulting from pedogenic transformation. The scarcity of the 2:1
type clay minerals was attributed to addition of ash and low rainfall.
The accumulation rate of soil organic matter was most rapid in ows
up to 2000 yr old, tending to zero after 15,000 yr. Stabilisation of organic
matter was related to pyrophosphate-extractable Al and Fe phases, ka-
olinite and clay content, rather than to imogolite-type material.
Baglieri et al. (2010) nd, however, that glycerol-extractable soil organ-
ic fractions were closely bound to SROMS, particularly in older
(N 9000 yr) soils (Udivitrands), and consequently less subject to biodeg-
radation. From their rst analysis of ve Vitric Andosols (IUSS Working
Group WRB, 2006) in an elevation sequence (551 to 1772 m) on 10,000
to 15,000 yr BP lava, on the north-east ank of Mt Etna, Egli et al. (2007)
conclude that organic C stocks, (and imogolite-type minerals and crys-
talline Fe-oxyhydroxides) decreased with elevation.
In a study of their elevation sequence of soils, Egli et al. (2012) ad-
dress the complex and important issue of re impacts on the soils. The
stable soil organic fraction at higher elevations survived from
N 8000 yr, dating close to the onset of pedogenesis, whereas with de-
creasing elevation soil charcoal analysis indicates a history of increased
re frequency causing vegetation change and rejuvenation of soil or-
ganic matter. The authors conclude, however, that re frequency on
Mt Etna is not such as to deplete a large pool of soil organic matter. In
a subse quent s tudy of this elevation sequence, Ma
strolonardo et al.
(2013) illustrate the importance of re on the quality and quantity of
soil organic matter on Mt Etna. Fire frequency is inferred to be a factor
in stocks of organic C and N and of C resistant to chemical oxidation
(all increasing with decreasing elevation). These authors stress the
major importance of vegetation and local climate as factors in amounts
and nature of the organic matter in the soils they analyse.
Bellanca et al. (1996) account for differences between A an d B hori-
zons in one soil to an age-difference of tephra between the horizons. In
the present study we nd boundaries between buried tephra units
difcult or impossible to identify, and no morphologically distinct bur-
ied soil was found. Buried units of tephra may be dened by detailed
grain size and chemical analyses of soil proles (Taboada et al., 2007),
but there are few data on buried soils in tephra sequences of Mt Etna
(Zehetner et al., 2003). Superimposed buried soils (classied as Typic
Hapludands) in two deep exposures of radiocarbon-dated tephra
sequences are described by Agnelli et al. (2007). These, at 1350 m a.s.l.
(S ank) and 1550 m a.s.l. (E ank), are now under chestnut
(Castanea sativa) and pine forest (Pinus laricio), respectively. The base
of the oldest sequence sampled dates from ~7000 yr BP, the timescales
for developmen t of the buried soils being much shorter than this,
b 800 yr in one case. Neoformed amorphous and crystalline mineralog-
ical phases were absent, a feature attributed to insufciently intense, or
too short a period, of pedogenesis.
4. Materials and methods
Soils were sampled at 23 sites in 1999. Sites (Fig. 1a) cover a range in
age of subjacent lava ow (72 yr to ~35,000 yr), elevation, vegetation
and agriculture. The soils examined occur in teph ra (of varying
particle-sizes) deposited on lava and accumulating particularly in cracks
and depressions, though in two soils alluvial deposition of local soil ma-
terial was evident. Below, soils are referred to by site number with age
of ow in brackets. The ages of lava ows (Supplementary Table 1)
are based on the Geological Map of Etna Volcano ( Branca et al., 2011).
The dating of lavas older than reliable historic records (N~400 yr BP)
is tentative and where possible based on archaeomagnetic and radio-
metric dating (Tanguy et al., 2012), though dates for some older ows
are constrained only by stratigraphical relationships. For the statistical
analysis of the soils the age of ow for each site is recalibrated against
a baseline of 2000 AD. For those lavas constrained only by a range of
age (in some cases, very broad), a date at the mid-point of the range is
used (Supplementary Table 1). Archaeological evidence (Drs. Gianna
Ayala and Francesco Privitera, pers. comm.) was used in the eld
where possible. Where surface and margins of older
o
ws are concealed
by well developed plant cover and where agricultural use is intensive it
may be difcult to identify ows in the eld, or to know the age of the
underlying lava. ES23 posed a particular problem, this being a dagala
2
of unknown age (Supplementary Table 1). Samples from this site are
therefore excluded from all statistical analyses which involve the age
of ow.
Annual rainfall for study sites (Supplementary Table 1) is estimated
by interpolation from data quoted by Durbin and Henderson-Sellers
(1981) (Fig. 1b). Sites and soils were described in the eld, following
Schoeneberger et al. (2012). Where feasible, soil pits were dug to bed-
rock or to clast-supported lava blocks (horizons denoted by R in Sup-
plementary Table 1). As tephra stratication was impossible to dene
in most cases (c.f. Buurman et al., 2004), the horizon notation used in
Supplementary Table 1 gives the pedological character of each horizon
but does not dene sequences of buried layers, which inevitably occur
(Agnelli et al., 2007). As we do not have all data required for full classi-
cation following Soil Taxonom y (Soil Survey Staff, 1999), we name
soils tentatively at the Order or Suborder level only. The three criteria
for Andisol classication not determined are P adsorption, bulk density
and volume proportion of volcanic glass (Soil Survey Staff, 1999). The
high glass content observed in many relatively young soils on Mt Etna
(which we classify as Entisols) will reduce the threshold amounts of
SROMs required to qualify a soil as A ndi sol (Soil S u rvey Staff, 2014 ), but
on this criterion alone, ES1(1928 AD); ES2(1928 AD); ES3(1928 AD);
ES9(1809 AD); ES12 (1651 AD) and ES22(1843 A D) would fail to qual ify
as Andisols (year of emplacement of lava ow in parentheses after site
number).
Prior to laboratory analysis, samples were air-dried, disaggregated
and passed through a 2 mm sieve. Sub-samples for particle-size analysis
were not dried further in order to avoid irreversible aggregation on dry-
ing where amorphous materials are present (Nanzyo et al., 1993). Laser
281P. James et al. / Catena 137 (2016) 277297
diffraction was used for particle-size determination of b 1mmmaterial
in preference to the sieve and pipette method as it was considered pru-
dent to keep samples in suspension for as short a time as possible. Tex-
tural classes named in Supplementary Table 1 are based on estimation
in the eld (following Mizota and van Reeuwijk, 1989). We infer the na-
ture of neoformed materials from chemical data rather than from direct
physical examination. For phases of Al, Fe and Si conventional subscripts
are used, as shown in Table 2. The conventional procedures were used
for extraction of : 1. organic-bound (by pyrophosphate), denoted
below as e.g. Fe
p
; 2. inorganic non-crystalline (by NH
4
oxalate), denoted
as e.g. Fe
o-p
, and 3. crystalline phases of Fe and Al (by dithionite), denot-
ed as e.g. Fe
d-o
. These data are to be treated with caution as, for example,
oxalate may dissolve a portion of the crystalline phase (García-Rodeja
et al., 2007; Agnelli et al., 2007; Algoe et al., 2012).
In the following sections, after discussion of prole morphology, the
analytical variables are examined in relation to age of ow and location
in terms of ank of the volcano, the latter varying signicantly in rainfall
amount and tephra fallout from summit eruptions. For brevity, the
moister anks of the volcano (i.e. NE, E, SE and S) are referred to as E
ank, other sectors as W ank. Spearman correlation (ρ =coefcient;
p = probability of null hypothesis) and the MannWhitney U test are
used. For parametric statistical analyses non-normal variables are trans-
formed using log
10
, square root, and in one case (Fe
d-o
) an inverse trans-
formation. Of the functions tested for regression models relating
transformed soil variables to age of ow (Figs. 2 and 10a) the quadratic
proved efcient in terms of signicance level in the great majority of
cases . Multivariate analyses used are principal components analysis
(pca) and cluster analysis. In the former, orthogonal components are ro-
tated using Varimax. The unsupervised, agglomerative, hierarchical
cluster analysis using the rst four components of the pca is based on
the Euclidean distance as the dissimilarity measure and Ward's group-
ing method, with all variables standardized to mean of zero and vari-
ance of 1.
5. Results
5.1. Prole morphology
Details of prole morphology and site conditions are given in Sup-
plementary Table 1. Table 3 groups soils according to extent of develop-
ment expressed in prole morphology. On this criterion, there is a clear
trend of increasing pedogenesis with age of ow between the groups, I
to IV (Table 3). Only in the two soils of group IV, ES6 (153.9 ka) and
ES13 (450 AD), was there a clear B horizon and a colour other than
black (with colour value 3). Very shallow soils, Psam ments (I,
Table 3), occur in rock-dominant areas of (mostly young) lava ows
with very thin (commonly b 5 cm) accumulations of tephra and other
allochthonous mineral and organic material in depressions in bedrock
surfaces or between blocks in aa lava. Vegetation is limited to lichens
or mosses, or may also include a narrow range of vascular plants. Exam-
ples are ES1, ES2 and ES3 on the 1928 Mascali ow. Organic matter con-
tent may be relatively high under a more developed plant cover
(Supplementary Table 1). The soils are black (2.5Y 2.5/1), sandy, cindery
or ashy, vitric, and occur in pockets (typically b 30cm in diameter) in ex-
posed lava where organic debris and small amounts of tephra collect.
Commonly, ash and lapilli are trapped within the plant cover. On aa
ows, mosses bridge narrow interstices between the lava clinker.
Smooth, little weathered surfaces of pa hoehoe lava on a 1991 ow
collect mineral and plant detritus, moisture and rabbit dung in depres-
sions, and support a number of plant species and larvae. The lava sur-
faces beneath these soils had no sign of weathering visible in the eld.
Vingiani et al. (2013) report that mineral fragments which have been
transported to accumulate in cavities and depressions on young lava
ows of Mt Etna are little altered chemically beneath a cover of lichens.
Developed in thicker tephra than those soils described above, are
coarse-textured Entisols (or Andisols) with limited visible evidence of
pedogenesis (II in Table 3). Sufcient amounts of glass, value of
(Al
o
Fe
o
)% and horizon thickness ( 18 cm) would qualify some of
these soils as Vitrandic Xeropsamments or Vitrandic Ustipsamments
(depending on moisture regime). Tephra is of highly variable thickness
(7 cm to N 120 cm), depending in part on the underlying lava morphol-
ogy. With the exception of reddish brown Monti Rossi (Red Mountains)
tephra in ES20 (1329 AD) the soils are black (predominantly 2.5Y 2.5/1
and 10YR 2/1), ashy, cindery or pumiceous, sandy, occasionally skeletal,
loose, stony or very stony, non-sticky and non-plastic. Such soils are
very common in the s tudy area on ows of all morphologies, dating
from 1923 AD to 1329 AD in the cases we sampled. They have O, A or
Ap horizons between 3 and 15 cm thick over little altered ash, lapilli,
cinder, clinker, pumice or scoria. The soils vary in texture, horizon thick-
ness and depth to lithic contact. Otherwise, variation in prole mo rphol-
ogy is limited to very weak, barely percep tible structure in some A
horizons. Organisms observed in these soils in February were larvae,
millipedes and ants. Roots commonly extend to bedrock. Vegetation in-
cludes herbs and grasses, often with mosses and, in some cases, lichens.
Broom is c ommon (to 4 m in height); deciduous oaks ar e 3 m h igh near
ES14 (1865 AD), 8 m near ES15 (1689 AD) and mature near ES21
(1408 AD); evergreen oak grows to 6 m at ES12 (1651 AD) and ES20
(1329 AD). Land has been organized for terraced agriculture only at
ES20 (1329 A D). Here, dark redd ish-brown (2.5YR 3/4) c ind er from the
1669 Monti Rossi ank eruption at the base of the soil appears to have
been used to level the lava surface for terrace construction. There was ev-
idence of grazing by sheep at ES9 (1809 AD) and by rabbits at other sites.
Other soils developed in tephra and lacking a B horizon are Entisols
or Andisols (III in Table 3)ofner texture than in groups I and II,
Table 2
Soil properties determined, techniques used, and units expressed. Except for particle-size fractions, b 2 mm material is analysed.
Soil properties Unit Method and description
Total Ca, Mg, K, Na mg g
1
Digestion in nitric and perchloric acids (Hesse, 1971); determination of Ca and Mg by atomic absorption spectrometer; of
Na and K by colorimeter using molybdenum blue method (Allen, 1989).
Particle size fractions
(in b 1 mm material)
% vol Coulter laser diffractometer, following pretreatment with H
2
O
2
, and dispersion with Calgon. Textural terms based on
eld estimation because of the unreliability of particle-size analysis of Andisols (Nanzyo et al., 1993).
Pyrophosphate-extractable Fe and Al
(Fe
p
,Al
p
)
mg g
1
Extracted with potassium pyrophosphate (McKeague, 1967). For Fe and Al present in metalorganic complexes.
Oxalate-extractable Fe, Al and Si
(Fe
o-p
,Al
o-p
,Si
o-p
)
mg g
1
Extracted in darkness with acid NH
4
-oxalate (McKeague and Day, 1966). Inorganic, non-crystalline.
DCB-extractable Fe (Fe
d-o
)mgg
1
Extracted with dithionitecitratebicarbonate (Mehra & Jackso n, 1960). Crystalline Fe oxyhydroxides (goeth ite, magnetite, etc.)
pH in water log [H+] Electrometrically in water (2.5:1 volume water:soil)
Organic carbon % weight Potassium dichromate digestion by the WalkleyBlack method (Hesse, 1971)
Organic nitrogen % weight Kjeldahl procedure (Allen, 1989)
Fig. 2. Soil variables plotted against log of age (x axis) of lava ow for all samples. Black circles: W ank sites; open circles: E ank. Curves for quadratic (or linear) function and 95% limits
shown in each case. R
2
=coefcient of determination. For samples with equal or very nearly equal values, symbols are over-written (e.g. samples in ES17 and ES18 in several cases).
282 P. James et al. / Catena 137 (2016) 277297
283P. James et al. / Catena 137 (2016) 277297
containing loamy sand or sandy loam. Age of ow varies from 1180 ±
30 AD (ES7) to 4228 ka (ES17 and ES18). Where exposed, lava mor-
phology includes slabby aa, pahoehoe, levé e and tumulus.
3
The soils
are black, with A or Ap and C horizons. The ner texture of these soils
results from either a ner parent tephra (ES4 153.9 ka) or from more
advanced weathering than in those soils described above. In the silty
clay loam of ES17 and ES18 (both in the range, 42 to 28 ka), no material
coarser than sand-size was observed. Related to the ner texture is a
weak granular or blocky structure in A horizons of some soils and the
presence of earthworms (Supplementary Table 1), which, where more
common, cause marked bioturbation: in ES17, earthworm burrows, ex-
tending to the base of the soil, are a striking feature. Consistence varies
from non- to moderately sticky, but slightly friable in ES8 (153.9 ka).
All sites have a mixed herb and grass ora; a number have been terraced
for agriculture, recent cultivation having been abandoned in several
cases. Trees are deciduous oak: 3 m high at ES7 (1180 ± 30 AD) and
6matES8(153.9 ka); olives (Olea eu ropaea)atES10(153.9 ka)
and olives and prickly pear (Opuntia cus-indica) in a walled enclosure
at ES4 (153.9 ka). In all cases where lava surfaces were examined be-
neath soils they were clean, irrespective of age of ow. Red tephra of
the Monti Rossi 1669 eruption occurred in ES24 (1000 ± 50 AD) (Sup-
plementary Table 1).
The most fully developed soils examined have a Bw horizon (IV,
Table 3). These are either Inceptisols or Andisols, are very dark greyish
brown (10YR 3/2) to brown (10YR 4/3), silt loam over clay loam (ES6
153.9 ka) and loam (ES13 1651 AD). There was no evidence of recent
tephra fall at these sites, both of which occur on the W ank of the vol-
cano. A horizons are blocky; the Bw in ES13 being blocky and in ES6,
prismatic. ES6 is distinct in most respects from other soils examined.
Unusually for Etnean soils, the lower pa rt of the prole is not free-
draining, slight gleying occurring on the ped faces of the lower part of
the Bw horizon. The soil, at least in part, appears to have been washed
into the depression where the soil occurs, which may account for the
presence of pottery fragments at a 3334 cm depth. ES6 and ES13 are
in enclosed elds and are the most intensively farmed soils examined,
ES6 being grazed by cattle and ES13 cultivated. Trees are 4 m-high de-
ciduous oak, almond (Prunus dulcis) and prickly pear at ES6, and pista-
chio (Pistacia vera) at ES13. There is a trend of increasing pedogenesis
with age of lava ow, as reectedinsoilprole morphology (with the
exception of ES6 and ES13), from I to IV. Soils are listed by age but with-
in each group it is not possible to separate soils on the basis of extent of
development.
5.2. Soil physical and chemical properties
Analytical data for all samples are presented in Supplementary
Table 2. Values for selected variables (for each soil sample) are plotted
against age of ow in Fig. 2, in which the E and W anks (as de
ned
in Se
ction 4) are differentiated. Where available estimates give a
broad range of age, the mid-point of the range is used (Section 4). In
Table 4a are shown correlations for variables with age and rainfall, re-
spectively; also listed (Table 4b) are variables which differ signicantly
(using the MannWhitney U test) between the W and E anks of the
volcano, with the ank noted on which the median of the variable is
greater.
5.2.1. Particle-size
Soil texture, as estimated in the eld, ranges from coarse sand to clay
loam. The size distribution and proportion of particles N 2mmvaryap-
preciably (Supplementary Table 1), tending to be of high concentration.
Fine particle-size fractions correlate strongly with age of ow, as illus-
trated for b.002 mm (clay) in Fig. 2a and by the envelopes of particle-
Table 3
Summary of general soil and site characteristics grouped into (tentative) soil Orders (I to
IV). There is a trend of increasing pedogenesis with age of lava ow (with the exception of
ES6 and ES13), from I to IV. Soils are listed by age but within each group it is not possible to
separate soils on the basis of extent of development.
Soil Age of
ow, yr
Broad soil charact eristics
ES1 72 I Very shallow, black Psamments or Orthents in pockets in
lava. Plant cover predominantly mosses and lichens; few
vascular plants.
ES2 72
ES3 72
ES11 77 II Black soils, including Entisols, Psamments (possibly
Andisols) in little-altered tephra. Soil depth and range of
soil mesofauna are greater than in group I, though no
evidence of earthworms was found. Occasionally very weak
structure in A horizons. Highly variable vegetation from
bryophytes and grasses to mature trees. Some agricultural
terracing.
ES14 135
ES22 157
ES9 191
ES19 208
ES15 311
ES12 349
ES21 592
ES20 671
ES7 820 III Black loamy Entisols, possibly Andisols; occasional weak
granular or blocky structure; earthworms. Vegetation
includes mature cropped trees associated with terraced
agriculture.
ES24 1000
ES5 1050
ES23 Unknown
ES8 ~9500
ES4 ~9500
ES10 ~9500
ES17 ~35,000
ES18 ~35,000
ES13 1550 IV Greyish-brown to brown, loamy and ne loamy Inceptisols;
possibly Andisols: blocky A (over prismatic Bw horizon in
ES6). ES6 is pasture; ES13 pistachio orchard.
ES6 ~9500
Table 4a
Spearman correlations with age of ow and rainf all for all samples and near-surface samples
only (only signicant correlations shown). Correlations signicant at 95% in bold. pc =
principal component score (Table 5). ES23 (dagala within 1780 AD ow) is omitted from
analyses involving age of ow.
All samples Near-surface samples only
Age Rain Age
b.002 mm ρ .760 .591 .868
p b.001 .000 .000
.002b.02 mm ρ .748 .561 .871
p b.001 b.001 b.001
Organic C ρ .272 .017
p .015 .879
Organic N ρ .445 .271
p b.001 .012
C/N ρ .455 .593 .422
p b.001 b.001 .050
pH ρ .324 .320
p .003 .003
Ca ρ .156 .388
p .166 b.001
Mg ρ .165 .258 .424
p .143 .018 .049
Na ρ .850 .379 .929
p b.001 b.001 b.001
K ρ .135 .601
p
232 b.001
Fe
p
ρ .711 .404 .531
p b.001 b.001 .011
Fe
o-p
ρ .275 .256
p .013 .019
Fe
d-o
ρ .719 .607 .853
p b.001 b.001 b.001
Fe
o-p
/Fe
d-o
ρ .286 .675 .509
p .010 b.001 .016
Al
p
ρ .607 .489 .598
p b.001 b.001 .003
Al
o-p
ρ .314 .272 .449
p .005 .012 .036
Si
o-p
ρ .170 .107
p .131 .331
pc1 ρ .766 .449 .862
p b.001 b.001 b.001
pc4 ρ 247
p .027
pc5 ρ .256
p .022
284 P. James et al. / Catena 137 (2016) 277297
size curves in Fig. 3. Silt, .002b.02 mm, also correlates strongly with age
(Table 4a). The b.002 mm fraction correlates negatively with rainfall
(Table 4a), there being a signicant difference in b.002 mm material be-
tween the E and W anks (Table 4a), the lowest concentrations occur-
ring across the SE and S anks (Fig. 4). Despite intermittent addition
of tephra to many of the soils, for near-surface samples only, the corre-
lation between b.002 mm and age of ow is higher (ρ = .868; p b .001)
than that for all samples (Table 4a). For the deepest samples taken in
each soil, the correlation with age of ow is ρ =.781,pb .001. The sig-
nicant correlation with age of ow for near-surface samples obtains for
the variables listed in Table 4a, the degree of correlation being high in a
number of cases.
As allochthonous particles have been discounted as a signicant
component of the soils of Mt Etna (Section 2), we attribute the increase
in the ner fractions to pedogenesis. The map of proles of b.002 mm for
each soil (Fig. 4)reects age-dependency and factors related to location
on the volcano. Intense weathering in organic-rich, near-surface hori-
zons is a likely cause of an up-prole increase in b.002 mm material
(Fig. 4) in a number of ows up to 1050 yr BP (ES5 950 ± 50 AD; ES9
1809 AD; ES15 1689 AD; ES21 1408 AD; ES22 1843 AD and ES24
1000 ± 50 AD), but more marked is a down-prole increase in older
ows (partic ularly in ES6 153.9 ka; ES13 1651 AD; ES17 4228 ka;
ES18 4228 ka) resulting from either of, or both, weathering of older
tephra at depth and weathering of lava at the lithic contact. Argillic ho-
rizons (Bt) which formed as a result of illuviation of clay are common in
well developed soils of the Mediterranean region (Ya
alon, 1997). Soils
with Bt horizons are mapped at lower elevations of Mt Etna by Fierotti
et al. (1988) (Fig. 1c), and they occur in older volcanic soils of southern
Italy (Lulli, 2007). We have not undertaken micromorphological analy-
ses, but nd no evidence indicating clay illuviation in the soils we
sample.
5.2.2. Organic carbon
The marked near-surface enhancement of organic matter in many of
the soils (Fig. 5) is the most visible morphological effect of pedogenesis
across the full age-range of ows sampled, and on all anks of the vol-
cano. Organic C concentration is nowhere of sufcient depth to form a
melanic epipedon. Charcoal may be a cause of black coloration in volca-
nic soils, but in the soils we examine the colour is determined by little-
weathered, bas altic parent mater ial. Owing to the sensitivity of soil
organic matter concentration to plant cover, dilution by tephra
and other disturbance, amoun ts of organic C vary widely. Hi gh con-
centrations (N 8.0%) occur in surface horizons beneath a range of
vegetation types: cult ivated fruit tr ees (ES 4 153.9 ka: 8.96%);
mature deciduous oak (ES21 1408 AD: 9.8%); discontinuous herb
and grass cover in a lava levée (ES5 950 ± 50 AD: 8.3%), and beneath
the lichen of ES2 (1928 AD: 8 .8%). There is no distinct regional
pattern in organic C proles (Fig. 5), and organic C does not correlate
with rainfall (Table 4 a). There is, however, a weak but signi cant
correlation be tween organic C and age of ow (ρ = .263, p = .016;
Fig. 2b). In their sample of soils Egli et al. (2008) nd a steady
increase of soil organic C stock to an age of about 15,000 yr.
Hopkins et al. (2007) also nd that more developed soils have higher
concentrations of organic C (and N) on Mt Etna.
Table 4b
Signicant differences (95% level) between W and E anks, using MannWhitney U.
Signicant differences
U Flank with greater median
Fe
d-o
1354.5 W
Fe
o-p
/Fe
d-o
169 E
Fe
o-p
456 E
Fe
p
1157.5 W
Al
p
1171 W
Si
o-p
600.5 E
C/N 328 E
.021000 mm 339 E
.002- b 20 mm 1185 W
b.002 mm 1256 W
b.001 mm 1241 W
K 1249 W
Ca 503 E
Na 461 E
Mg 576 E
pH 1065.5 W
pc1 1162 W
pc3 386 E
pc4 521 E
Not signicant
Organic C
Organic N (p = .53)
Al
o-p
pc2
Fig. 3. Envelopes of cumulative particlesize curves (.0011 mm) for soils on ows
grouped by age: a. 72 to 208 yr BP: ES1, ES2, ES3, ES11, ES14, ES22, ES9 and ES19. b. 311
to 1050 yr BP: ES15, ES12, ES21, ES20, ES7, ES24 and ES5. c. ~9000 to ~35,000 yr BP:
ES4, ES6, ES8, ES10, ES13, ES17 and ES18. ES23 age unknown.
285P. James et al. / Catena 137 (2016) 277297
Fluctuations in organic matter content with depth would be expect-
ed in soils which have received intermittent tephra deposits or have
been disturbed in some other way. In the majority of soils sampled
there is, however, a progressive decline in organic matter content
with depth (Fig. 5). In the moderate concentrat ions of organic C at
depth in the agricultural terrace soil, ES8 (153.9 ka) (4.47% at 23 cm
depth, and 5.59% at 53 cm), there is a suggestion of soil burial resulting
from terrace construction.
The quality of organic matter in terms of organic N and the ratio, C/N,
is more closely related to age of ow than is organic C (evident in the
correlations, Table 4a). The possible inuence of rainfall amounts on or-
ganic matter quality, though not organic C, is also evident in correlations
with organic N and C/N (Table 4a), and also in the signicant EW ank
differences in C/N but not in organic C (Table 4a). Organic N just fails the
95% level for signicance of this difference. Lower C/N ratios in Andisols
of drier xeric as compared with udic moisture regimes may be attribut-
ed to a higher rate of mineralization in the former (cf Broquen et al.,
2005).
5.2.3. pH
pH varies between 4.8 and 7.7, many values being higher than those
reported by Shoji et al. (1993) for volcanic soils in other climatic regions,
higher than reported by Certini et al. (2006) for the largely trachydacitic
Mt Amiata, Tuscany, but comparable with values found by Vacca et al.
(2003) on Roccamonna and by Egli et al. (2008) on Mt Etna (5.1 to
6.25). Most of the values b 5.5 occur on the E ank (Fig. 6), presumably
reecting acid deposition from the plume and higher rainfall. There is an
increase in pH with age (ρ .307, p .004) (Fig. 2c; Table 4a), which is more
marked with ES17 and ES18 (both 4228 ka) samples omitted for this
correlation: ρ =.610andp=b.001. It is not clear why this increase oc-
curs. The correlation between pH and rainfall is negative, and there is a
signicant pH difference between the E and W anks (Table 4b). There
are, however, variations on each ank with, for example, anomalously
low values which are difcult to explain in ES17 and ES18 (4228 ka)
(Fig. 2
c), samples ES17/1 to 3 yielding the only values b 5.
0.
5.2.4. Bases
In common with ndings of Egli et al. (2008), Na is the most respon-
sive of the bases to weathering and leaching. In the present study, of all
those measured, this property shows one of the most marked changes
with age of ow (correlation: ρ .850, p b .001), the rate of loss show-
ing no sign of abating over the ~35,000-yr range (Fig. 2d). The highest
values for Na (reaching 7.27 g kg
1
) occur in the Psammen ts, 11
(1923 AD) and ES22 (1843 AD) (Supplementary Fig. 1). The signicant
correlation with age for near-surface samples only is very high and neg-
ative for Na (Table 4a). At variance with the Na trend is the lack of
Fig. 4. Schematic map of clay: b.002 mm % against depth for each soil. Exact location of sample sites shown in Fig. 1a.
286 P. James et al. / Catena 137 (2016) 277297
correlation between age of ow and Ca and Mg, and lack of at least
linear correlation with age for K (Figs . 2eg; Table 4a). This contrasts
with the ndings of Egli et al. (2008),thatbasecationleachingisa
dominant process in their sample of Mt Etna so ils, and of Martínez
Cortizas et al. (2007), who report a reduction in K and Ca (as well
as in Na), with an increasing degree of weathering in a wide range
of European volcanic soils. Such element depletion Martínez
Cortizas et al. (2003) attribute to the diluting effect of organic mat ter
on element concent ration, and the mobility of certain elements.
Nevertheless, they nd that, despite sig nicant depletion of Ca and
K in soils of the Gauro (Campi Flegrei) and Vico volcanoes, Ca en-
hancement occurs in organic-rich horizons, which they attribute to
biocycling of the element. In most soils we analyse, Ca and more es-
pecially K show an up-prole increase (Supplementary Figs. 2 and 3),
these cat ions correlating negatively with d epth in the soil (Ca: ρ
.364, p .001; K: ρ .312, p .004). Ca correlates positively with organic
C(ρ .396, p b .001). Quality of organic matter (and therefore possibly
its age) appears, however, to have greater control on base distribu-
tion than organic C concentration: K correlates negatively with the
C/N ratio. Ca, Mg and Na (in contrast t o its negative correlation
with organic C) correlate positively with C/N ( correlation details
not shown). A factor in greater near-surface Ca, Mg and Na concen-
trations in a number of soils we sample may be soil-rejuvenation
by tephra. A signicant distinction o f Mt Etna among most of its
European volcanic peers is that it is continually active .
Na, Mg, Ca and K all correlate with rainfall (K negatively: Table 4a),
and all show signicant differences between the E and W anks, the
rst three tending to occur in g reater amount on the E ank (Table 4b).
The EW contrast may reect greater tephra fall on the E ank. This, how-
ever, wo uld not a ppear to b e a major factor in K distribution.
5.2.5. Extractable Fe
Concentrations of extractable Fe phases organic-bound (Fe
p
), non-
crystalline (Fe
o-p
) and cr ystalline (Fe
d-o
) are plotted for e ach sample in
Fig. 7. For the majority of samples, the relative abundance of the three
phases is in the order, Fe
o-p
N Fe
d-o
N Fe
p
, a feature of t he Etnean soils re-
ported by Fernández-Sanjurjo et al. (2003),andonereecting both the
status of the SROM, ferrihydrite, and the limited pe dogenesis in m uch of
the study area. T he stronger corr elations with age of ow, however, a re
for the crystalline (Fe
d-o
: ρ .719, p b .001) and organic-complexed (Fe
p
:
ρ .711, p b . 001) phases, bot h showing a steepening increase o n ows of
N 200 yr (Fig. 2 hj). The ratio, Fe
o
/Fe
d
, related to the crystallinity of the
Fe oxides (Schwertmann, 1985 ), has b een found to reect the degree of
pedogenesis in volcanic soils, the ratio value decreasing with age
(Malucelli et al., 1999). This trend i s indicated in our samples by a wea k
but signicant negative correlation between Fe
o-p
/Fe
d-o
and age of ow
Fig. 5. Schematic map of % organic carbon against depth for each soil. Exact location of sample sites shown in Fig. 1a.
287P. James et al. / Catena 137 (2016) 277297
(Table 4a). In contrast, on volcanic materials of Roccamonna Volcano
(older t han those w e analyse) Vacca et al. ( 2003) nd Fe
o
/Fe
d
not t o be
age-dependent. In our M t Etna samples, Fe c rystallisation is favoured by
seasonal dehydration in the Mediterranean climate (Schwertmann,
1985), which is drier on the W ank, where median F e
d-o
is signicantly
greater than on the E (Table 4b). The correlation between Fe
o-p
/Fe
d-o
and rainfall is quite strong, and t here is a signicant difference between
the E and W anks (Table 4b). Fe
o-p
/Fe
d-o
shows no consistent depth pat-
tern between soils, but a clear down-prole increase in several of the soils
(Supplementary Fig. 5: ES7 1180 ± 30 AD; ES19 1792 AD; ES23 age un-
known; ES21 1408 AD on the E ank, and ES12 1651 AD and ES17
35,000 bp on the W), the proportion of crystalline secondary Fe reecting
more advanced transformation in layers of older tephra, and/or less crys-
tallinity i n organic-rich ho rizons.
5.2.6. Extractable Al and Si
The presence of Al and Si in the form of SROMS is inferred from the
range of Al
o-p
(.26% to 2.44%) and Si
o-p
(.064% to 1.36%), the rst corre-
lating with age of ow, but not the latter (Table 4a; Figs. 2l and n). Lack
of correlation between each of these phases and organic C is not consis-
tent with the protection of organic matter by SROMS, commonly in-
ferred for volcanic soils, though degree of humication m ay be a
factor: Al
o-p
correlates negatively with C/N (ρ .315, p .004). As Fe
p
,
the organic-bound Al
p
increases with age, the Al and Fe organic phases
being quite strongly inter-correlated (ρ .787, p b .001). In most samples,
pH (N 5.0) is favourable to allophane and imogolite formation, and al-
though Al
p
increases with age, Al
p
/Al
o-p
is b 0.5 (a condition associated
with the presence of allophane and imogolite: Legros, 2013)inallsam-
ples except ES6/2 to 4 (153.9 ka) and 13/3 (450 AD). Al
p
and Al
p
/Al
o-p
correlate with organic C (Al
p
: ρ .449, p b .001; Al
p
/Al
o-p
: ρ .447, p b .001),
but both have negative correlations with C/N (Al
p:
ρ .432, p b .001;
Al
p
/Al
o-p
: ρ .315 p .004), reecting the ability of the humied fraction
to compete for Al, thereby reducing Al availability for the formation of
allophane.
The value of the qualifying criterion for Andisols, (Al
o
Fe
o
)%, in-
creases with age of ow (Fig. 2o), a number of samples in soils on
N 9000-yr ows having values N 2.0 (samples ES6/4 153.9 ka; ES8/2
and 3 153.9 ka; ES17/1, 2 and 3 4228 ka), but also the much younger
ES14 (1865 AD) (samples 3 and 4). However, high glass content in-
creases the potential for qualifying as Andisols on this criterion, a
value of (Al
o
Fe
o
)% = 0.4% being the minimum required (Soil
Survey Staff, 2014, p. 18). This value is exceeded in all samples analysed,
though in many, other qualifying criteria for Andisols which are not
measured are unlikely to be met.
Bellanca et al. (1996) found that NH
4
oxalate-extractable Al, Fe and
Si concentrations all increase down-prole in soils to the north of
Fig. 6. Schematic map of pH (H
2
O) against depth for each soil. Exact location of sample sites shown in Fig. 1a.
288 P. James et al. / Catena 137 (2016) 277297
Nicolosi. In our greater ra nge of soils, there is no consistent down-
prole trend in Al
o-p
or Si
o-p
(Figs. 8 and 9). The reason for higher
amounts of Si
o-p
occurring on the N side of the volcano is not clear.
6. Principal components analysis
The variables selected for the pca and the rotated component matrix
are shown in Table 5.Therst four components (pcs) with initial eigen-
values N 1 account for only 70% of the variance but give a meaningful in-
sight into combinations of soil variables reecting key dimensions of
pedogenesis on the volcano. Pc1 includes those properties shown to
be related to age of ow and which reect, and may be used as one in-
dicator of, the extent of pedogenesis as regards transformations involv-
ing Fe, Al and Si, the four bases analysed, organic matter and particle-
size distribution. High loadings obtain for measures of ne particle-
size, Na (negative), Fe
d-o
(inverse positive) and organic C. The highest
negative loading for C/N ratio in the matrix measures the greater degree
of humication associated with this component. The high loading for
Al
p
relative to the negative loading of Si
o-p
gives a non-allophanic signal
in the component, and reects the increase of Al
p
with age of ow.
Sample scores on pc1 correlate with age of ow and negatively with
rainfall (Fig. 10a; Table 4a), there being a signicant difference in scores
between the volcano's E and W anks (Table 4b), giving a marked re-
gionalization to scores; negative scores occurring on the SE and S sides
of the volcano (Fig. 10b), possibly reecting greater tephra addition on
these slopes. An up-prole increase in scores occurs in ES5 (950 ±
50 AD); ES12 (1651 AD) ; ES22 (1843 AD); ES4 (153.9 ka); ES9
(1809 AD); ES11 (1923 AD); ES7 (1180 ± 30 AD), ES15 (1689 AD);
ES23 (age unknown), ES20 (1329 AD); ES21 (1408 AD) and ES24
(1000 ± 50 AD) (Fig. 10b), indicating greater alteration, as dened by
this component, in near-surface horizons. This is contrary to the nd-
ings of Bellanca et al. (1996) regarding more intensive alteration in B
than in A horizons, referred to in Section 3.Adown-prole increase in
scores occurs , however, in four of the most developed soils (ES18
35 ka; ES13 450 AD; ES6 153.9ka;ES8153.9 ka).
In pc2, the two phases, Si
o-p
and Al
o-p
, are isolated with their high
scores consistent with an allophanic effect. The component lacks indica-
tors of advanced pedogenesis with loadings low for ne particle-sizes
and positive for Na. With Al
p
/Al
o-p
inserted into the analysis in lieu of
Al
p
and Al
o-p
(result not shown), the structure of components 1 and 2
is little changed, but the loading of Al
p
/Al
o-p
is high positive in pc1 and
moderate negative in pc2, reinforcing the distinction between condi-
tions associated with an allophanic versus a non-allophanic effect. Pc2
correlates signicantly with neither age of ow nor rainfall, nor is
there a signicant difference in scores between E and W anks. There
is no clear regionalization in pc2 scores. A down-prole increase in
scores occurs (not shown) in eleven soils: ES18 (4228 ka); ES4 (15
3.9 ka); ES6 (153.9 ka); ES8 (153.9 ka); ES 22 (1843 AD); ES14
(1865 AD); ES19 (1792 AD); ES20 (1329 AD); ES21 (1408 AD); ES23
(age unknown) and ES24 (1000 ± 50 AD), which may relate to decreas-
ing organic matter content with depth.
pc3 is a moderate Fe
o-p
and positive (inverse) Fe
d-o
component, the
resulting ratio between these phases suggesting the role of ferrihydrite.
The component does not correlate with age, but positively with rainfall
(ρ .523, p b .001: Table 4a), and there are signicantly greater scores on
the E ank (Table 4b), consistent with conditions favouring persistence
of a high Fe
o-p
/Fe
d-o
ratio. The less organic-rich soil at depth may explain
a predominant down-prole decrease in scores (not shown), occurring
in all soils except ES6 (153.9 ka), ES12 (1651 AD), and ES22 (1843 AD).
A feature of pc3 scores not shared by pc1 or pc2 is a reduction in rate of
decrease down-prole, or a reversal in trend, in the lowest parts of the
sampled proles at twelve locations, ES4 (153.9 ka); ES5 (950 ±
50 AD); ES6 (153.9 ka); ES8 (153. 9 ka); ES9 (1809 AD); ES12
(1651 AD); ES14 (1865 AD); ES19 (1792 AD); ES20 (1329 AD); ES21
(1408 AD); ES23 (age unknown) and ES24 (1000 ± 50 AD), though
scores remain negative.
pc4, a relatively high C/Norganic Cbase cation component, is the
only component with positive loadings for all four bases, Ca, Mg, Na and
K. These are not associated in this component with signals of much ad-
vanced pedogenesis in terms of particle-size, Fe
o-p
/Fe
d-o
and C/N ratio.
Across the matrix, loadings differ between the bases , presumably
reecting the interplay between parent material, loss from the soil dur-
ing pedogenesis and retention by plants, the most salient features being
the marked loss of Na and a putative retention of at least K by plants. pc4
scores do not correlate with age; they are generally higher on the E ank
(Tabl e 4b), but there is no distinct regional pattern in scores (not
shown). A down-prole decrease occurs in scores of all soils except
ES11 (1828 AD), reecting the distribution of organic matter. The reduc-
tion in rate of decrease, or reversal of trend, at depth in the prole found
in pc3 scores occurs also in scores on pc4, with the exception of ES10
(153.9 ka); ES11 (1923 AD); ES12 (1651 AD); ES15 (1689 AD) and
ES17 (4228 ka). There is insufcient evidence to speculate on the rea-
son for this common, if su btle, change in conditions at depth in the
prole.
In pc5 a high positive loading for pH contrasts with much lo wer
loadings for other variables. The low % variance explained by this com-
ponent suggests a limited impact of pH in distinguishing between the
soils analysed, a reec tion possibly of a narrow range of pH across
most of the samples. The low but positive loading for pH in pc1 reects
the (unexplained) statistically signicant increase of pH with age of
ow described above.
Fig. 7. ThreeFephasesforeachsoilsample,mgg
1
:Fe
p
(black); Fe
o-p
(light grey) and Fe
d-o
(dark grey). Soils in order of increasing age of ow. Position of ES23 age unknown is
arbitrary.
289P. James et al. / Catena 137 (2016) 277297
7. Cluster analysis
Five clusters of samples are chosen from the analysis based on the
rst four pcs (Fig. 11), clusters 1 and 2 being a division of a group of
thirty-nine samples. The distribution of scores on the rst four pcs is
shown for each cluster in Fig. 12. The spatial distribution of sample
members in the ve clusters (Fig. 13) comprises a form of soil map for
the sites sampled. In cluster 1 both median and lower quartile are pos-
itive on all ve pcs, providing a pedogenic mixed message in which sig-
nals of relatively advanced formation occur with expressions of a strong
presence of Al, Si and Fe SROMS (with highest mean of the clusters for
Al
o-p
,Si
o-p
and Fe
o-p
: 10.8, 5.3 and 12.3 m g g
1
,
respectively), and high
amounts of organic matter characterised by high C/N. Cluster 1 mem-
bers occur on all anks, especially in the NE, and in soils ranging from
135 yr to ~9500 in age (Fig. 13). In this cluster, the range in age and cli-
mate of sites, and the positive median score for all four pcs, give an im-
pression of a complex of controls on pedogenesis leading to relatively
similar outcome for all member samples.
Cluste r 2 (19 samples) is distinguished by the highest pc3 scores
(the high Fe
o-p
/Fe
d-o
high Mg pc) of all clusters. Sample scores on the
remaining pcs are largely negative. With the exception of ES5 (950 ±
50 AD), this cluster does not occur on the west ank (Fig. 13), cluster
members oc curring at sites with the highest rainfall (i.e. 750 to
1250 mm ).
In Cluster 3 (20 samples), scores on pc1 and pc2 are largely negative.
The cluster is distinguished by the highest median on pc4 (the high or-
ganic matterhigh C/Nhigh base pc). Samples occur on the E, SE and S
anks and in the younger soils of the W ank (ES221843 AD and
ES121651 AD).
Cluster 4 (9 samples) has the lowest scores of all clusters on pc1, and
mainly positive scores on the allophanic pc2. The cluster has the lowest
distribution of scores on pc3, (the ferrihydrite pc). The weakly devel-
oped samples, which occur in ES11(1923 AD), ES22(1843 AD) and
ES9(1809 AD), comprise the coarsest tephra (highest .021.0 mm %;
lowest b.002 mm %) and the highest Na amount (mean: 5.2 mg kg
1
)
of all clusters.
Cluster 5 (19 samples) is distinguished by its high positive median
on pc1 and its largely negative scores on other pcs. Unlike other samples
in this cluster, those in the upper 45 cm of ES17 (4228 ka) have high
positive score on the high Si
o-p
Al
o-p
component, pc2. Samples in this
cluster, which express a relatively well developed character, occur on
the oldest ows with the least rainfall, on the W ank; though single
samples occur anomalously near the surface of ES4 (153.9 ka), ES5
(950 ± 50 AD) and ES9 (1809 AD).
Fig. 8. Schematic map of Al
o-p
:mgg
1
against depth for each soil. Exact location of sample sites shown in Fig. 1a.
290 P. James et al. / Catena 137 (2016) 277297
8. Discussion
8.1. Soil processes and proles
The chief processes inferred from our data are largely those in-
ferred in a number of previous studies of Mt Etna, particularly that
of Egli et al. (2008). However, although loss of Na is one of the
most marked features of our sequence of soils, we do not, in contrast
to Egli et al. (2008) and Busà et al. (1998), have evidence of loss of Ca
or K. In our sample, the latter are associated with surface, organic-
rich hori zons and retained in the soil, a feature reected in pc4,
which subsumes high amounts of organic matter and hig h C/ N ratios
together with positive loadings for all bases. Other chief processes
we infer are: production of clay- and silt-size material; incorporation
of organic matte r and its modication in terms of decreasing C/N
ratio; the formation of SROMS (indic ated by NH
4
-extraction); devel-
opment of soil st ructure, and a tendency for vigorous bioturbation in
ner g rained, older soils. We do not identify allophane directly, but
the pca clearly distinguishes conditions favouring allophane forma-
tion. A protective mechanism relating amou nts of organic C to NH
4
-
extractable phases is not borne out in the d ata. The degree of
humication (C/N ratio) of organic mat ter, in a ddition to its
Fig. 9. Schematic map of Si
o-p
:mgg
1
against depth for each soil. Exact location of sample sites shown in Fig. 1a.
Table 5
Principal components analysis: loadings of variables and % of variance explained. NB: in-
verse transformation used for Fe
d-o
.
Rotated component matrix
Component
12345
Log Fe
p
.907 .124 .014 .116 .007
Log .002b.02 mm .904 .069 .226 .062 .010
Log Na .896 .131 .018 .243 .043
Log b.002 mm .895 .004 .225 .149 .105
Sqrt Al
p
.805 .274 .274 .098 .072
Inverse Fe
d-o
.752 .184 .332 .197 .258
Fe
o-p
.591 .042 .481 .249 .359
Log Si
o-p
.247 .891 .146 .019 .095
Sq rt. Al
o-p
.398 .850 .020 .170 .065
K .202 .005 .853 .094 .021
Mg .203 .190 .626 .268 .082
Ca .051 .189 .226 .724 .052
Sqrt org. C .553 .005 .053 .679 .046
C/N .472 .146 .121 .492 .072
pH .114 .123 .066 .026 .955
% variance explained 37.7 11.8 11.6 10.34 7.70
291P. James et al. / Catena 137 (2016) 277297
concentration, emerges as a signicant inuenceonsoilchemistry.
An up-prole increase in pc1 scores in many soils indicates greater
near-surface intensity in certain major processes.
Soils of limited development, generally with distinct organic-rich A
horizon in some cases overlain by an O horizon, but lacking B horizon,
occur on ows dating up to 4228 ka. These organic-rich horizons and
Fig. 10. a. pc1 scores plotted against log of age of lava ow (x axis) for all samples. Black circles: west ank sites; open circles: east ank. Linear best-t and 95% limits shown. R
2
=co-
efcient of determination. For samples which have equal or near-equal values, symbols are over-written (e.g. in ES17 and ES18). b. Schematic map of pc1 scores against depth for each
soil. Vertical line = score of 0 in each case. Exact location of sample sites shown in Fig. 1a.
292 P. James et al. / Catena 137 (2016) 277297
a weak structure in the A horizon are the most distinct pedogenic ef-
fects. With increasing age of ow, a ner texture occurs and bioturba-
tion is more in evidence. Only two soils have Bw horizons, one (ES6
153.9 ka) formed in uv ially deposited volcanic material, the other
(ES13 450 AD), a rich soil in a mature agricultural landscape. We do
not verify the presence of illuviated clay using micromor phological
techniques, but we nd no suggestion of illuviation having taken
place. Nor are we able to quantify secondary crystalline clay minerals
in the soils.
In one agricultural terrace there is a buried soil (ES8 153.9 ka), and
two pr oles (ES6 153.9 ka and ES17 153.9 ka) contain inwashed
sediment, but there is no clear evidence of major disruption of prole
development in other soils, despite the identication of buried tephra
units in two of the proles. With the exception of Mg (Supplementary
Fig. 4), which we are not able to explain, many properties measured
do not have very irregular depth functions which would be expected
in soils comprising tephra layers of differing age. The near-surface en-
hancement of organic matter, which occurs in all but one soil, is typical
of proles which have developed on relatively stable surfaces.
8.2. Environmental factors
Although the soils do not form a true chronosequence, their relation-
ship with age will be formed in part by the oldest material in each soil,
regardless of addition of fresh tephra. At least as important as ow-age
in determining soil character and distribution on the volcano are rainfall
and other, unmeasured, environmental contrasts between windward
and lee sides of the volcano. The size of Mt Etna and direction of prevail-
ing wind have major implications for the volcano's soil geography. Mt
Etna is unique in its combination of volcanic activity and local climates.
On the E and SE anks, rainfall amounts should favour pedogenesis, yet
this appears to be restrained (as reected in low pc1 scores), possibly by
tephra depos ition . A complex interpla y between effects of ow-age,
tephra deposition and location on the volcano is reected in the map
of cluster membership of samples (Fig. 13).
Lulli (2007) concludes that the Mediterranean climate of volcanic re-
gions of Italy does not favour allophane genesis and the full develop-
ment of Andosols, but conditions favourable to allophane formation
on Mt Etna are indicated in our analyses, and Andisols are classied in
a number of studies referred to in Section 3. Soils with illuviated clays
were mapped on Mt Etna by Fierotti et al. (1988), and a reddened B ho-
rizon was reported in a Typic Hapludand in late Pleistocene pyroclastic
material at 838 m a.s.l. on Vico Volcano, Italy (Quantin et al., 1985), but
in the soils we examine, even on the drier, more Medite rranean W
ank, and on the 4228 ka ow, neither red dening nor evidence of
clay illuviation was found. This contrasts with Pleistocene volcanic
soils elsewhere in the Mediterranean region: reddened soils with pedo-
genic calcite and illuviated clay in Pleistocene andesites of Methana,
Gree
ce, (m.a.r. b 400 mm; James et al., 1997) and on Quaternary basalts
of Morocco (Hamidi et al., 1999; Dekayir and El Maâtaoui, 2000); and
reddened soils in Pleistocene scoria of the Golan Heights, Israel (m.a.r.
850950 mm: Singer et al., 2004).
8.3. Conclusion
We demonstrate the difculties which may face the application of
chronosequence models to soil studies in complex volcanic environ-
ments. Our sample of soils is spread across a range of ecological, climatic
and agricultural environments. Ages of prehistoric lava ows are only
broadly constrained. For all ows, known or estimated age gives only
the date of onset of lava-weathering and related pedogenesis. At un-
known intervals, tephra has contributed soil parent material, this
forming the bulk of soils we examine. Two of the soils contain material
inferred to be washed from surrounding ows. The sample of soils
therefore comprises only an approximate chronosequence. Soil variabil-
ity within single ows is not examined explicitly, though, for example,
Fig. 11. Dendrogram from cluster analysis using sample scores on four principal compo-
nents. Cluster numbers 1 to 5 are as in Figs. 12 and 13.
293P. James et al. / Catena 137 (2016) 277297
the contrast between ES17 and ES18 (on the 2842 ka ow) in terms
at least of sedimentology and soil thickness illustrates the signi-
cance of lava-ow reli ef for so ils. We infer certain mineralogical
properties from chemical analyses, themselves arguably appr oxi-
mate. The classication of soils is only partial, and, owing to the ex-
actitude of modern soil classic ation s chemes, wo uld requi re
determination of further soil properties to complete. Whatever the
ner classes to which Mt Etna's soils may be allocated, however,
the majority of those we sample are entirely, and inevitably, volca-
nic in their appearance and major properties.
The heuristic approach we take is chiey statistical, with subjective
interpretation of spatial patterns of selected properties. Our sample of
soils is small, in view of Mt Etna's diverse environments. Nevertheless,
the number of signicant correlations, corroborated in many cases by
geographical distribution, appears to indicate a causal relationship be-
tween a number of soil properties and, respectively, age of lava ow
and rainfall (and presumably other important environmental contrasts
between the volcano's anks). This nding echoes the conclusion of
Colombo et al. (2007) with respect to volcanic soils of south-central
Italy, that such factors as duration of pedogenesis, human impact and
soil climate appear to play more important roles than effects of stratied
pyroclastic materials. Not investigated in the present study is the impact
of vegetation on pedogenesis. The importance of plant cover is evident
from previous research into the soils of Mt Etna (Section 3) and is
reected in the general relationship between vegetation and age of ow
(Supplementary Table 1). Patterns we are not able to explain include
the increase of pH with age of ow and the higher Si
o-p
and Al
o-p
concen-
trations o n the N slope.
That age of ow emerges as a chief soil-forming factor in the com-
plexity of the Etn ean environment may be remarkable. Puzzling,
however, is the correlation between age of ow and a number of
soil properties for near-surface samples only. This ts easily wi th
neither retardant nor developmental effects (sensu Johnson,
1985) of intermittent tephra deposition on soil prole developme nt.
The complexity of the effects of tephra addition remains a challenge
for research into the soils of Mt Etna. So too do processes at the base
of soils and the contribution of lava-bedrock weathering to soil pro-
les. The soil-rock interface remains a largely unexplored soil envi-
ronment on the volca no.
Supplementary data to this article can be found online at http://dx.
doi.org/10.1016/j.catena.2015.09.023.
Acknowledgements
The authors are grateful to the Royal Geographical Society (HSBC
Small Grant) for covering part of eldwork costs; to the Universities of
Fig. 12. Box plots showing median and upper and lower quartiles of scores on each of the four pcs for each cluster. Each pc is identied on the y axis.
294 P. James et al. / Catena 137 (2016) 277297
Liverpool and Bedfordshire; to Sandra Mather for producing the illustra-
tions; to Alan Henderson, Irene Cooper and Bob Jude for the laboratory
analyses; to Drs. Gianna Ayala and Francesco Privitera for the archaeo-
logical data, and to the three referees for their constructive comments.
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